In general, an AC/DC converter for converting an AC current to a DC current is needed in order to drive a brushless DC motor (hereinafter simply referred to as “motor”) with an AC power source. The DC current outputted from the AC/DC converter is converted to a three-phase drive current by an inverter circuit. The motor is rotated by supplying the drive current to the motor.
The AC/DC converter is provided at the outside of the motor in some cases and inside the motor in other cases. The external electric power is often supplied in the form of an AC current. For that reason, a motor that can be rotated by supplying the AC current is preferred in some cases.
In case where an AC power is supplied to the motor, it is necessary to arrange the AC/DC converter inside the motor as stated above. However, in case where a DC power is supplied to the motor, the AC/DC converter becomes useless. Moreover, the size of the motor grows larger in proportion to the size of the AC/DC converter.
In the meantime, a demand has existed for a motor whose rotation speed can be adjusted with a digital speed limiting signal. To this end, a microcomputer needs to be installed within the motor. It is necessary to install the microcomputer at the upstream side of an inverter drive circuit. However, if the microcomputer is mounted on an electric circuit board arranged within the motor, the electric circuit board grows larger in size and the size of the motor also becomes larger.
In view of the above, the brushless DC motor that can be driven with the AC power and whose rotation speed can be adjusted by a digital speed limiting signal becomes larger in size. Moreover, if the DC power is supplied to the motor, the unnecessary AC/DC converter leads to increase in the cost and size of the motor.